As Buddhist externalists, both Saṅghabhadra and Śubhagupta claim the existence of an external object on the basis of atomism. In this paper, I will show the interrelationship between Saṅghabhadra’s and Śubhagupta’s atomic theories. Regarding the ontological status of the aggregation of atoms, both of them agree on a Vaibhāṣika principle that the aggregation of atoms, as a real substance, can serve as an object-support of cognition. Based on this principle, their similarities can be further explicated from three aspects. Regarding epistemology, (...) Śubhagupta differs from Saṅghabhadra on the cognitive process of the awareness of something blue. For Saṅghabhadra, a gross object is grasped by non-conceptual sensory consciousness because it is a real entity aggregated by atoms. Through the function of vitarka of sensory consciousness, an object with its essential nature, i.e., the colour blue, is distinguished from other entities. Then, it is known as the notion ‘blue’, which is a mere provisional existence, through the conceptual thought of mental consciousness. However, for Śubhagupta, a coarse object such as something blue is only a mental error of conceptual construction. (shrink)